The present invention relates to the field of lasers. More particularly, the present invention relates to the field of attenuating laser beams.
There are a variety of reasons for characterizing a laser. One means for characterizing a laser beam is to use CCD""s (Charge Coupled Devices). Many laser beams of interest are too powerful for direct observation by the CCD""s. A laser beam attenuator reduces the power of such laser beams so that the beams can be characterized by the CCD""s. Laser beam attenuators of the prior art include absorptive and reflective neutral density filters, multilayered dielectric filters, high energy variable attenuators, diffractive beam splitters, and polarizing variable attenuators.
The absorptive neutral density filters include an optically absorptive material which is formed within an optically transparent material. The optically absorptive material absorbs part of the incident laser beam so that a transmitted laser beam is attenuated. Due to the absorptive nature of the absorptive neutral density filters, absorptive neutral density filters are capable of attenuating low power laser beams only. Also, absorptive neutral density filters induce wavefront distortion and polarization effects.
The reflective neutral density filters include a metallized partial reflector on an optical substrate. The laser beam is incident upon the metallized partial reflector, which partially reflects and partially transmits the remaining portion of the laser beam. The partially transmitted laser beam forms the attenuated laser beam. Though reflective neutral density filters, they also induce the wavefront distortion and polarization effects. The reflective neutral density filters attenuate higher power laser beams than the absorptive neutral density filters but are not feasible for attenuating high power laser beams having high power densities.
The multilayered dielectric filters include alternating layers of a high index of refraction material and a low index of refraction material. These alternating layers are deposited on an optical substrate. The multilayered dielectric filters reflect part of the laser beam and transmit the remaining part of the laser beam. The transmitted laser beam forms the attenuated laser beam. Some multilayered dielectric filters are highly wavelength dependent. So such filters must be used with specific wavelength laser beams. Also, the multilayered dielectric filters induce the wavefront distortion. Further, the multilayered dielectric filters are expensive, lack precision, and are not feasible for attenuating high power laser beams having high power densities.
The polarizing variable attenuators include first and second polarizers. The first polarization angle of the first polarizer is at an angle of less than 90xc2x0 to the second polarization angle of the second polarizer. The laser beam is attenuated by absorption in the first and second polarizers. By allowing the first polarization angle to be varied with respect to the second polarizer, the polarizing variable attenuators are made variable. Due to the absorptive nature of the polarizer pairs, the polarizer pairs are limited to attenuating low power laser beams. Further, the polarizing variable attenuators are highly polarization dependent.
The high energy variable attenuators include first and second sets of parallel optical plates. The parallel optical plates attenuate the laser beam by Fresnel reflections from each successive optical plate. Thus, the transmitted laser beam forms the attenuated laser beam. By varying an incidence angle, the high energy variable attenuators are made variable. The second set of parallel plates is oriented such that a beam deviation introduced by the first set of parallel plates is cancelled by the second set of parallel plates. The high energy variable attenuators are highly polarization dependent. The high energy variable attenuators are typically limited to an attenuation of about 40 dB. Also, the high energy variable attenuators induce the wavefront distortion since some reflected light recombines with the transmitted laser beam due to additional reflections.
What is needed is a laser beam attenuator having low wavefront distortion, good power handling, a large fixed attenuation value, a low polarization effect, and low wavelength dependence.
The present invention is a laser beam attenuator and a method of attenuating a laser beam. The laser beam attenuator includes first and second prisms, a beam dump, and a light absorbing body. An input laser beam partially refracts and partially reflects at a first surface of the first prism to form first refracted and reflected laser beams. The first reflected laser beam partially refracts and partially reflects at a second surface of the second prism to form second refracted and reflected laser beams. The beam dump and the light absorbing body absorb the first and second refracted laser beams. Thus, the second reflected laser beam forms an attenuated laser beam. An alternative laser beam attenuator uses the first prism. In the alternative laser beam attenuator, the first reflected laser beam forms an alternative attenuated laser beam.